Gaseous waste products resulting from the combustion of hydrocarbonaceous fuels, such as gasoline and fuel oils, comprise carbon monoxide, hydrocarbons and nitrogen oxides as products of combustion or incomplete combustion, and pose a serious health problem with respect to pollution of the atmosphere. While exhaust gases from other carbonaceous fuel-burning sources, such as stationary engines, industrial furnaces, etc., contribute substantially to air pollution, the exhaust gases from automotive engines are a principal source of pollution. Because of these health problem concerns, the Environmental Protection Agency (EPA) has promulgated strict controls on the amounts of carbon monoxide, hydrocarbons and nitrogen oxides which automobiles can emit. The implementation of these controls has resulted in the use of catalytic converters to reduce the amount of pollutants emitted from automobiles.
In order to achieve the simultaneous conversion of carbon monoxide, hydrocarbon and nitrogen oxide pollutants, it has become the practice to employ catalysts in conjunction with air-to-fuel ratio control means which functions in response to a feedback signal from an oxygen sensor in the engine exhaust system. Although these three component control catalysts work quite well after they have reached operating temperature of about 300° C., at lower temperatures they are not able to convert substantial amounts of the pollutants. What this means is that when an engine and in particular an automobile engine is started up, the three component control catalyst is not able to convert the hydrocarbons and other pollutants to innocuous compounds.
Adsorbent beds have been used to adsorb the hydrocarbons during the cold start portion of the engine. Although the process typically will be used with hydrocarbon fuels, adsorbent beds can also be used to treat exhaust streams from alcohol fueled engines. The adsorbent bed is typically placed immediately before the catalyst. Thus, the exhaust stream is first flowed through the adsorbent bed and then through the catalyst. The adsorbent bed preferentially adsorbs hydrocarbons over water under the conditions present in the exhaust stream. After a certain amount of time, the adsorbent bed has reached a temperature (typically about 150° C.) at which the bed is no longer able to remove hydrocarbons from the exhaust stream. That is, hydrocarbons are actually desorbed from the adsorbent bed instead of being adsorbed. This regenerates the adsorbent bed so that it can adsorb hydrocarbons during a subsequent cold start.
The prior art reveals several references dealing with the use of adsorbent beds to minimize hydrocarbon emissions during a cold start engine operation. One such reference is U.S. Pat. No. 3,699,683 in which an adsorbent bed is placed after both a reducing catalyst and an oxidizing catalyst. The patentees disclose that when the exhaust gas stream is below 200° C. the gas stream is flowed through the reducing catalyst then through the oxidizing catalyst and finally through the adsorbent bed, thereby adsorbing hydrocarbons on the adsorbent bed. When the temperature goes above 200° C. the gas stream which is discharged from the oxidation catalyst is divided into a major and minor portion, the major portion being discharged directly into the atmosphere and the minor portion passing through the adsorbent bed whereby unburned hydrocarbon is desorbed and then flowing the resulting minor portion of this exhaust stream containing the desorbed unburned hydrocarbons into the engine where they are burned.
Another reference is U.S. Pat. No. 2,942,932 which teaches a process for oxidizing carbon monoxide and hydrocarbons which are contained in exhaust gas streams. The process disclosed in this patent consists of flowing an exhaust stream which is below 800° F. into an adsorption zone which adsorbs the carbon monoxide and hydrocarbons and then passing the resultant stream from this adsorption zone into an oxidation zone. When the temperature of the exhaust gas stream reaches about 800° F. the exhaust stream is no longer passed through the adsorption zone but is passed directly to the oxidation zone with the addition of excess air.
U.S. Pat. No. 5,078,979, issued Jan. 7, 1992 to Dunne, which is incorporated herein by reference in its entirety, discloses treating an exhaust gas stream from an engine to prevent cold start emissions using a molecular sieve adsorbent bed. Examples of the molecular sieve include faujasites, clinoptilolites, mordenites, chabazite, silicalite, zeolite Y, ultrastable zeolite Y, and ZSM-5.
Canadian Patent No. 1,205,980 discloses a method of reducing exhaust emissions from an alcohol fueled automotive vehicle. This method consists of directing the cool engine startup exhaust gas through a bed of zeolite particles and then over an oxidation catalyst and then the gas is discharged to the atmosphere. As the exhaust gas stream warms up it is continuously passed over the adsorption bed and then over the oxidation bed.
U.S. Pat. No. 5,744,103, issued Apr. 28, 1998 to Yamada et al., discloses a hydrocarbon adsorbent for engine exhaust gas cleaning. The adsorbent contains large pore zeolites having 12+ membered rings (“MR”), smaller pore zeolites having 8 MR and in-between pore zeolites having 10 MR. Disclosed examples of the zeolites are those having the topologies (as identified by the International Zeolite Association (“IZA”)) FAU (e.g., zeolite Y), AFY and Beta (i.e., 12 MR zeolites); CHA (8 MR); and MFI (e.g., ZSM-5), MEL and FER (10 MR).
U.S. Pat. No. 5,603,216, issued Feb. 18, 1997 to Guile et al., discloses reducing the amount of hydrocarbons emitted during engine start (cold start) using two zones in the exhaust system using the same or different zeolite adsorber(s) in each zone. The zeolite(s) may be small pore zeolite which adsorbs low molecular weight alkenes (ethylene and propylene) and large pore zeolite which adsorb higher molecular weight hydrocarbons (e.g., pentane). Disclosed examples of zeolites are ZSM-5, Beta, gmelinite, mazzite, offretite, ZSM-12, ZSM-18, Berryllophosphate-H, boggsite, SAPO-40, SAPO-41, Ultrastable Y, mordenite and combinations thereof.
Elangovan et al., Journal of Physical Chemistry B, 108, 13059-13061 (2004) discloses the zeolite designated SSZ-33 (a zeolite having intersecting 10 and 12 MR pores with a large void at the intersections) for use as a hydrocarbon trap to reduce cold start emissions. The performance of the SSZ-33 is compared to that of Beta, Y, mordenites and ZSM-5 zeolites. SSZ-33 is said to have superior performance over Beta, Y, mordenites or ZSM-5.
U.S. Patent Application Publication 2005/0166581, published Aug. 4, 2005 by Davis et al., discloses molecular sieves used as adsorbents in hydrocarbon traps for engine exhaust. The method comprises contacting the exhaust gas with molecular sieves having the CON topology (per the IZA). The CON molecular sieve can be used by itself or can be used with another adsorbent. Disclosed examples of CON molecular sieves are those designated SSZ-33, SSZ-26, and CIT-1. ITQ-4 is also disclosed, but it is believed ITQ-4 has the IFR topology, not the CON topology. Disclosed examples of the other adsorbent are molecular sieves designated SSZ-23, SSZ-31, SSZ-35, SSZ-41, SSZ-42, SSZ-43, SSZ-44, SSZ-45, SSZ-47, SSZ-48, SSZ-53, SSZ-55, SSZ-57, SSZ-58, SSZ-59, SSZ-60, SSZ-63, SSZ-64, SSZ-65 and mixtures thereof.